1. Technical Field
The present invention relates to a method of producing a semiconductor wafer and a semiconductor wafer.
2. Description of the Related Art
In recent years, Group 3-5 compound semiconductors such as GaAs, AlGaAs, and InGaAs have been used to produce electronic elements such as a field effect transistor (FET), a high electron mobility transistor (HEMT), and heterojunction bipolar transistor (HBT). To produce such electronic elements, a compound semiconductor epitaxial wafer is used. The compound semiconductor epitaxial wafer is produced by growing a crystal of a Group 3-5 compound semiconductor on a semi-insulative wafer such as a GaAs wafer by an epitaxial growth method. The epitaxial growth method includes liquid-phase epitaxy, molecular beam epitaxy, and metal organic chemical vapor deposition (MOCVD).
Japanese Patent Application Publication No. 11-345812 discloses a compound semiconductor epitaxial wafer including an AlGaAs buffer layer between a semi-insulative GaAs wafer and an n-type GaAs active layer. The buffer layer suppresses a leakage current, which degrades the characteristics of a FET. The buffer layer also suppresses the influence of the wafer or the impurities on the wafer on the characteristics of the FET. The buffer layer disclosed in Japanese Patent Application Publication No. 11-345812 is formed by metal organic vapor phase epitaxy (MOVPE), and donor impurities and acceptor impurities whose concentrations are close to each other are added to the buffer layer.
Japanese Patent Application Publication No. 2007-067359 discloses a Group 3-5 compound semiconductor device including a p-type buffer layer that is formed by MOVPE. Japanese Patent Application Publication No. 2007-067359 takes note of the relation between the thickness of the p-type buffer layer and the p-type carrier concentration of the p-type buffer layer, and proposes that the leakage current of the Group 3-5 compound semiconductor device be reduced by setting the product of the thickness and the p-type carrier concentration to range from 1×1010 to 1×1012 (cm−2).
Although Japanese Patent Application Publication No. 11-345812 does not disclose the crystal growth conditions of the buffer layer, a normal method of forming a Group 3-5 compound semiconductor by MOVPE or MOCVD involves supplying the Group 5 sources such as P and As significantly more than the Group 3 sources such as Al, Ga, and In. This resultantly increases the cost of producing the compound semiconductor epitaxial wafer. Japanese Patent Application Publication No. 2007-067359 discloses that the p-type carrier concentration of the p-type buffer layer, which is doped with oxygen or a transition metal, is controlled. Japanese Patent Application Publication No. 2007-067359, however, does not consider how many amount of the Group 5 sources are supplied.
To cut the production cost, it is preferable to reduce the amount of the Group 5 sources to be supplied. If a reduced amount of the Group 5 sources is simply supplied in order to cut the production cost, however, the p-type carrier concentration of the Group 3-5 compound semiconductor becomes excessively high. Consequently, excess acceptor impurities cannot be ionized and thus remain. Accordingly, the Group 3-5 compound semiconductor cannot behave sufficiently as a buffer layer.
Specifically speaking, when MOVPE or MOCVD is employed, organometallic compounds such as trimethylgallium and trimethylaluminum are supplied as the Group 3 sources. The carbon in the organometallic compounds is incorporated into the crystal of the compound semiconductor during the crystal growth. The carbon concentration of the Group 3-5 compound semiconductor increases as the ratio of the Group 5 sources to the Group 3 sources, during the crystal growth, decreases. Since the carbon behaves as the acceptor impurities in the crystal of the Group 3-5 compound semiconductor, the p-type carrier concentration of the Group 3-5 compound semiconductor increases as the carbon concentration increases. Consequently, the Group 3-5 compound semiconductor cannot behave sufficiently as a buffer layer.
More specifically, if the p-type carriers remain in the Group 3-5 compound semiconductor, the residual capacitance of the Group 3-5 compound semiconductor increases, which resultantly increases the leakage current of the Group 3-5 compound semiconductor. This lowers the withstand voltage of the Group 3-5 compound semiconductor. This also degrades the carrier mobility in a semiconductor device such as a FET to be formed in the Group 3-5 compound semiconductor.
To prevent the excess acceptor impurities that cannot be ionized from remaining in the Group 3-5 compound semiconductor so that the Group 3-5 compound semiconductor can behave sufficiently as a buffer layer, it is preferable to maintain the p-type carrier concentration of the Group 3-5 compound semiconductor at an appropriate level while reducing the amount of the Group 5 sources to be supplied. Therefore, it is an object of the present invention to provide a method of producing a Group 3-5 compound semiconductor, according to which the use of the Group 5 sources can be reduced without degrading the properties of the Group 3-5 compound semiconductor.